Both signaling conducted via the enhanced physical downlink control channel (E-PDCCH) and machine type communication (MTC) or machine-to-machine (M2M) may utilize only a small portion of the overall usable radio resource space. In particular, MTC may require simultaneous transmission of very small allocations from a large number of mobile terminals. The E-PDCCH has been developed to, for example, avoid physical downlink control channel (PDCCH) capacity limitations for systems with multi-user (MU)—multiple input multiple output (MIMO) and coordinated multiple point (CoMP) transmission/reception, and also for potentially better inter-cell interference coordination by moving the control to legacy physical downlink shared channel (PDSCH) regions.
Control and other information may be transmitted via the downlink via both the E-PDCCH and PDSCH such that the control and other information transmitted via the E-PDCCH and the PDSCH may be multiplexed, such as by frequency division multiplexing (FDM). In this context, FDM means that both slots of a physical resource block (PRB) pair can be utilized for E-PDCCH, e.g., typically there is no time division multiplexing (TDM) of E-PDCCH and PDSCH in the same PRB pair. From this perspective of multiplexing, E-PDCCH will be similar to PDSCH, e.g., multiplexed to a number of PRB pairs. This principle is illustrated in FIG. 1 where an example allocation of 1 PRB for E-PDCCH is FDM-multiplexed with PDSCH. As noted by the cross-hatching in FIG. 1, the first slot of the subframe represents the Release 8/9 legacy PDCCH region, which may not be present in all cases. The PRB pairs allocated for E-PDCCH may be signaled by higher layers, e.g., Radio Resource Control (RRC), to the mobile terminal.
With FDM multiplexing, the number of resource elements (REs) available for E-PDCCH within one PRB pair may become rather high. In this context, an RE is one orthogonal frequency division multiplexing (OFDM) symbol—subcarrier pair within the OFDM transmission grid. The following table shows the number of REs available for E-PDCCH with different numbers of REs allocated for common reference symbols (CRS) and PDCCH and with either 12 or 24 REs allocated for demodulation reference symbols (DM-RS) that are used for demodulation of E-PDCCH.
#PDCCH#CRS01230132/120NANANA1124/112 114/102102/90 90/782116/104108/9696/8484/724108/96 100/8892/8080/68Since Release 8 PDCCH may use only 36 REs, the foregoing table indicates that the number of REs used for one E-PDCCH may be too large and may therefore cause resource inefficiency. Hence, multiple E-PDCCHs may be multiplexed within the PRB pair.
Similar issues may be created with PDSCH transmissions in instances in which very small packets are transmitted to multiple mobile terminals. In this regard, one PRB may be too large since 64 quadrature amplitude modulation (QAM) and MIMO with multiple parallel data streams are supported. Hence in such a case PDSCH may be resource inefficient unless multiple PDSCHs are multiplexed within one PRB pair.
Any typical multiplexing method, e.g., FDM, TDM, code division multiplexing (CDM) or space division multiplexing (SDM), may be applicable for multiplexing multiple E-PDCCHs or PDSCHs within one PRB pair. However, reference signals should be provided for demodulation of the E-PDCCH or PDSCH. In one technique, one of the existing DM-RS ports, e.g., antenna ports 7, 8, 9 or 10, may be mapped to each part of the PRB pair, hence transmitting reference signals from as many DM-RS ports as there are parts of PRB pair. Alternatively, the reference signal resource elements transmitted from one DM-RS port may be distributed among the parts of the PRB pair.
Interference aware receivers such as linear minimum mean squared error-interference rejection combining (LMMSE-IRC) receivers, typically estimate the covariance matrix of the interference at reference symbol (RS) locations by using the method of residuals. In this regard, the interference covariance matrix may be determined by averaging sample estimates at RS locations after the own cell contribution, that is, the RS symbol itself, has been subtracted, e.g., by using channel estimates and pilot sequence knowledge. However, techniques for splitting the PRB pair and a related DM-RS association may result in different parts of the PRB pair facing different interference conditions than what is experienced at the DM-RS locations that are used to estimate the corresponding interference covariance matrix, such that the interference covariance matrix may be inaccurate.
FIG. 2 illustrates an example of the mismatch between the interference experienced by the PRB pair and the DM-RS locations. Relative to FIG. 2, two PRB pair parts that are FDM-multiplexed are shown with the first part #1 being associated with DM-RS port 7, and the second part #2 being associated with DM-RS port 8. The mobile terminal of this example may first estimate the channel and interference covariance from the corresponding DM-RS port using all 12 samples within the PRB, and then use these estimates for equalizing and suppressing interference when decoding the control information and/or data from the associated part of the PRB pair. Unfortunately, however, interference covariance estimated from DM-RS port 7 may, for example, be a sum of interference from DM-RS port 8 and outer-cell interference plus noise. Additionally, the interference covariance needed for proper equalization of control information and/or data is a sum of outer-cell interference plus noise, e.g., interference from the other DM-RS port is not present, since the PRB pair part #1 and PRB pair part #2 are FDM multiplexed and thus do no create interference to one another. As a result, the interference may be incorrectly estimated, thus impacting the performance of interference suppression and demodulation itself.
In Release 8 PDCCH, the control channels are transmitted within an aggregation of control channel elements (CCEs), where one CCE is a set of 36 REs mapped in a distributed manner over the whole system bandwidth. The control channels can be transmitted within 1, 2, 4, or 8 CCEs, called the aggregation level. The different numbers of CCEs enable link adaptation for PDCCH as a base station will be able to control the coding rate by selecting the number of CCEs for a given mobile terminal and downlink control information (DCI) format that is appropriately based on, for example, channel quality indicator (CQI) reports received from the mobile terminal. The concept of CCEs and related link adaptation mechanisms are expected to carry over to E-PDCCH such that each E-PDCCH will be transmitted within a concatenation of a certain number, e.g., 1, 2, 4 or 8, of control channel elements. However, there needs to be some flexibility in the resource mapping for E-PDCCH since E-PDCCHs will be transmitted to multiple mobile terminals in the same subframe so as to provide multiple access capability. On the other hand, the mobile terminal has no prior knowledge of the resource mapping before starting to decode E-PDCCH. At the same time, the E-PDCCH may be link-adapted so the mobile terminal also does not know the coding rate, e.g., aggregation level.
In the case of Release 8 PDCCH, search space and blind decoding are utilized to address these issues. In this regard, the mobile terminal performs several blind attempts to decode PDCCH with different assumptions about the coding rate, resource mapping and length of the DCI format. The mobile terminal determines that the PDCCH has been successfully decoded in the event that the CRC check passes, which also validates the assumptions regarding the coding rate, e.g., aggregation level, resource mapping and length of DCI format, etc. Additionally, search space is the set of different CCE starting positions, e.g., resource mapping, and different aggregation levels that the mobile terminal is to check for a possible PDCCH transmission. Accordingly, in instances in which the search space is ill-defined, the mobile terminal may be subjected to inefficient interference suppression.